Not applicable.
Not applicable.
1. Field of the Invention
This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a protective coating system for a thermal barrier coating on a gas turbine engine component, in which the protective coating system is resistant to infiltration by contaminants present in the operating environment of a gas turbine engine.
2. Description of the Related Art
Hot section components of gas turbine engines are often protected by a thermal barrier coating (TBC), which reduces the temperature of the underlying component substrate and thereby prolongs the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Air plasma spraying (APS) has the advantages of relatively low equipment costs and ease of application and masking, while TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures can be produced using other atomic and molecular vapor processes.
To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC system is typically limited by a spallation event driven by bond coat oxidation and the resulting thermal fatigue. In addition to the CTE mismatch between a ceramic TBC and a metallic substrate, spallation can be promoted as a result of the TBC being contaminated with compounds found within a gas turbine engine during its operation. Notable contaminants include such oxides as calcia, magnesia, alumina and silica, which when present together at elevated temperatures form a compound referred to herein as CMAS. CMAS has a relatively low melting eutectic (about 1190xc2x0 C.) that when molten is able to infiltrate to the cooler subsurface regions of a TBC, where it resolidifies. During thermal cycling, the CTE mismatch between CMAS and the TBC promotes spallation, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. Another detriment of CMAS is that the bond coat and substrate underlying the TBC are susceptible to corrosion attack by alkali deposits associated with the infiltration of CMAS.
Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are U.S. Pat. Nos. 5,660,885, 5,871,820 and 5,914,189 to Hasz et al., which disclose three types of coatings to protect a TBC from CMAS-related damage. These protective coatings are classified as being impermeable, sacrificial or non-wetting to CMAS. Impermeable coatings are defined as inhibiting infiltration of molten CMAS, and include silica, tantala, scandia, alumina, hafnia, zirconia, calcium zirconate, spinels, carbides, nitrides, silicides, and noble metals such as platinum. Sacrificial coatings are said to react with CMAS to increase the melting temperature or the viscosity of CMAS, thereby inhibiting infiltration. Suitable sacrificial coating materials include silica, scandia, alumina, calcium zirconate, spinels, magnesia, calcia and chromia. As its name implies, a non-wetting coating is non-wetting to molten CMAS, with suitable materials including silica, hafnia, zirconia, beryllium oxide, lanthana, carbides, nitrides, silicides, and noble metals such as platinum. According to the Hasz et al. patents, an impermeable coating or a sacrificial coating is deposited directly on the TBC, and may be followed by a layer of impermeable coating (if a sacrificial coating was deposited first), sacrificial coating (if the impermeable coating was deposited first), or non-wetting coating. If used, the non-wetting coating is the outermost coating of the protective coating system.
While the coating systems disclosed by Hasz et al. are effective in protecting a TBC from damage resulting from CMAS infiltration, further improvements would be desirable.
The present invention generally provides a protective coating system and method for protecting a thermal barrier coating (TBC) on a component used in a high-temperature environment, such as the hot section of a gas turbine engine. The invention is particularly directed to a protective coating system that significantly reduces if not prevents the infiltration of CMAS into the underlying TBC.
The protective coating system of this invention comprises inner and outer alumina layers and a platinum-group metal layer. The inner alumina layer is deposited on the thermal barrier coating, the platinum-group metal layer is deposited on the inner alumina layer, and the outer alumina layer is deposited on the platinum-group metal layer, so that the platinum-group metal layer is encased between the inner and outer alumina layers. The outer alumina layer is intended as a sacrificial layer that reacts with molten CMAS, forming a compound with a melting temperature that is significantly higher than CMAS. As a result, the reaction product of the outer alumina layer and CMAS resolidifies before it can infiltrate the TBC. The platinum-group metal layer is believed to serve as a barrier to infiltration of CMAS into the inner alumina layer and, therefore, the TBC. Notably, the inner alumina layer beneath the platinum-group metal layer appears to enhance the ability of the platinum-group metal layer to prevent infiltration of CMAS. In other words, the platinum-group metal layer is better able to perform as a barrier to CMAS infiltration if it is deposited on an alumina layer than if it were deposited directly on the TBC.
In view of the above, the protective coating system of this invention is able to increase the temperature capability of a TBC by reducing the vulnerability of the TBC to spallation and the underlying substrate to corrosion from CMAS contamination. The layers of the protective coating system can be preferentially deposited on limited surface areas of a component more susceptible to CMAS contamination. In this manner, the additional weight and cost incurred by the protective coating system can be minimized. Finally, the protective coating system of this invention can be applied during the process of rejuvenating a TBC on a component returned from field service, thereby further extending the life of a TBC.
Other objects and advantages of this invention will be better appreciated from the following detailed description.